1. Field of the Invention
The invention relates to a process for preparing bis(aminophenyl)alkanes (alternatively referred to as diaminodiphenylalkanes herein), especially bis(aminophenyl)methane (alternatively referred to as diaminodiphenylmethane herein).
2. Description of the Background Art
The preparation of bis(aminophenyl)alkanes by condensation of an aromatic amine such as aniline and aldehyde over acidic catalysts is known.
In the reaction, first of all, N-alkyl compounds are formed from an aromatic amine (aniline) and aldehyde. These preliminary condensation products then undergo further reaction, in the presence of acidic catalysts, to form aminals. These aminals subsequently undergo rearrangement, under the action of an acidic catalyst, to form bis(aminophenyl)alkanes.
The reaction produces various isomers of the bis(aminophenyl)alkanes. Mixtures of 2,2′-, 2,4′- and 4,4′-bis(aminophenyl)alkanes are formed. Also formed, as by-products, are higher condensation products having three to six nuclei, and also N-alkyl compounds.
According to the known state of the art, the bis(aminophenyl)alkanes obtained are processed further to form diisocyanates such as, for example, diisocyanato-dicyclohexylmethane or diisocyanatodiphenylmethane, or other diisocyanates. These compounds are important paint base materials and base products for, for example, the preparation of polyurethanes.
In the prior art, bis(aminophenyl)alkanes have frequently been prepared from the condensation of aniline and aldehyde, especially aniline and formaldehyde. In that reaction, according to its particular variant, either first the condensation product of aniline and formaldehyde has been prepared and has then been subjected to rearrangement in the presence of acids such as hydrochloric acid, for example, or else the condensation itself has been carried out in the presence of acids, under rearrangement conditions.
A disadvantage of this approach is that, in the case of homogeneous catalysis with mineral acids, salt-containing wastewaters are produced which are formed on neutralization of the acids. Furthermore, the aqueous mineral acids lead to corrosion problems in the production plants. Consequently, in further prior art, processes have been developed which use corresponding heterogeneous catalysts. In these processes, in addition to acidic ion-exchangers, acidic synthetic or natural silicon oxides or aluminium oxides are used as well, such as zeolites or clay minerals.
In U.S. Pat. No. 4,294,981, in a process of this kind, the condensation is carried out in the presence of a strong aqueous acid, the acid being subsequently removed by solvent extraction. The rearrangement is carried out in turn in the presence of a strong acid, which is used in a relatively small amount. Diatomaceous earth, clays or zeolites can be used as catalysts in this reaction stage.
DE-A-12 30 033 describes a process for preparing bis(aminophenyl)alkanes. It uses silicon-containing clay, a synthetic silicon dioxide-aluminium oxide catalyst or a magnesium oxide-aluminium oxide catalyst.
Another reaction process for preparing bis(aminophenyl)alkanes is described in DE-A-14 93 431. The catalyst it uses is silicon dioxide, silicon dioxide-aluminium oxide or acid-treated aluminium oxide. Preference is given to silica gel or bentonite-like clay which contains silicon dioxide and aluminium oxide and is preferably acid-activated.
U.S. Pat. No. 4,071,558 describes a preparation process for preparing bis(aminophenyl)alkanes wherein an acid-activated clay catalyst, a silicon dioxide-aluminium oxide-containing cracking catalyst or a silicon dioxide-magnesium oxide catalyst is used.
U.S. Pat. No. 4,039,580 describes a preparation process in which the condensation of aniline and formaldehyde is performed in the absence of a catalyst and then the condensation product is subjected to further reaction in the presence of diatomaceous earth, clays or zeolites to form bis(aminophenyl)methane. Similar reactions are also described by U.S. Pat. No. 4,039,581.
The catalysts from the group of magnesium oxides or aluminium oxides, clay catalysts or silicon dioxide catalysts have not become established, owing to their high prices, low activities, inconsistent quality and inadequate service lives.
In the more recent prior art, therefore, the proposal is made to prepare bis(aminophenyl)alkanes using as the catalyst an ion-exchanger which possesses acidic groups. EP 0 043 933 A1, for instance, describes a process for preparing polyamine mixtures having a high fraction of 4,4′-diaminodiphenylmethane and a low fraction of 2,4′-diaminodiphenylmethane, in which the catalyst used is an ion-exchanger based on a divinylbenzene/styrene copolymer. This ion-exchanger possesses sulphonic acid groups, a specific surface area of 2 to 40 m2/g and a pore size of 0.5 to 40 nm. Acidic groups used for the catalyst are sulphonic acid groups. The yields in this process are situated in the range from 60% to 78%. With the sulphonated styrene-divinylbenzene copolymer catalyst it is possible to prepare diaminodiphenylmethanes which possess a high 4,4′-diaminodiphenyl-methane content. The latter isomer is needed in particular for further processing, namely for reaction to form corresponding diisocyanates of the diphenylmethane series, which represent the starting materials in the preparation of polyurethanes or are used as paint base materials. The publication further describes how the fraction of 2,2′- and 2,4′-diisocyanatodiphenylmethane compounds must be extremely low, since for numerous fields of application in the polyisocyanate sector these isomers are unwanted. According to the prior art EP 0 043 933, the resulting diaminodiphenylmethane compounds are immediately subjected to a phosgenation to prepare corresponding diisocyanates.
The process described in EP 0 043 933 for preparing bis(aminophenyl)alkanes has the disadvantage that it possesses low yields and that, in spite of a high reaction temperature, very long reaction times are needed in order to achieve an industrially acceptable yield. A further disadvantage is that the prior-art process produces only a small fraction of 2,4′ isomer.
As well as aromatic isocyanates, the corresponding aliphatic isocyanates are particularly important in certain speciality sectors.
The next stage in the operation of preparing aliphatic isocyanates is the hydrogenation of the aromatic ring of the bis(aminophenyl)alkanes.
The hydrogenation of diaminodiphenylmethane produces, from the 4,4′ isomer, the 4,4′ trans/trans-, cis/cis- and cis/trans-diaminodicyclohexylmethane (PACM). The trans/trans-4,4′-diaminodicyclohexylmethane content has a considerable influence on the crystallization propensity of the diisocyanate. If the trans/trans 4,4′ fraction of the diisocyanate product prepared from PACM by phosgenation or other processes is too high, the diisocyanatodicyclohexylmethane may form crystals even at room temperature, which is a hindrance to its further processing to form polyurethanes. Prior to the further processing, therefore, costly and inconvenient process steps must be undertaken in order to reduce the 4,4′ isomer content to an acceptable level, so that crystallites are no longer formed. This is usually accomplished by increasing the concentration of the 2,4′ isomer.
A further requirement concerning the isomer content in the preparation of diaminodiphenylmethane is that there must be as small as possible a fraction of 2,2′ isomer present, since this isomer causes chain termination in the polymerization reaction during the subsequent step of processing to form polyurethanes. In order to avoid this additional cost and complexity, it is important, for this reaction route, to obtain a defined isomer ratio during the preparation of the diaminodiphenylmethane itself.
According to the state of the art to date, this isomer ratio is obtained by the costly and inconvenient procedure of purifying and distilling the diaminodiphenylmethane in order to be able to provide the isomers in the ratio needed for further processing.